I would recommend having experience working in customer service or being very good at dealing with people and being able to demonstrate that in an interview. The three most important skills you need as a canvasser are the ability to communicate issues in a simple way, interpersonal skills, and emotional control (can you or can you not deal with difficult people).
If you can do all of that you would make a great canvasser I think. Even if you can't, if you can demonstrate a desire to develop theses skills and show excitement about the political issues you should get the job. I think most people who get interviewed get hired to do what is called an "observation day" anyways, so don't worry about it. The observation day is like a second interview where you actually do an hour of work after observing a field manager do their job for four hours. If it is work that you find you enjoyed and gave it your best effort you should get the job.
Prepare your public speaking skills! Be friendly, energetic, and make EYE CONTACT. Sociability is stressed at Clean Water Action, as they want their employees to be able to comfortably talk to strangers and convince those strangers of the importance/need of donating to the organization. The interview is actually pretty easy, they really just look to see that you can adequately speak without saying "uh" "um" or "ahh" too often, and that you can look someone in the face and tell them what you think is correct. Mean what you say. -Be a fan of the environment and understand that global warming is progressing at an alarming rate today.